He was 18th in completion percentage, behind guys like Jason Campbell, Chad Henne, David Garrard and Joe Flacco
18th in passing yardage, again behind Campbell, Flacco, Garrard, etc.
Tied for 15th in TD passes
20th in yards per attempt behind the likes of Sanchez, Vince Young, Campbell, Garrard, Flacco
16th in passer rating behind more of the usual suspects
Then of course the playoff game where he went 18 of 36 for 146 yards and a 58.3 rating.
Which would all be fine if we were talking about a guy who was a rookie, but it's Carson Palmer who was being paid like a top 5 QB.
You are correct, when compared to others he was mediocre. I was just looking at him alone, which would make them decent stats.